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Cellular forums Home > Archive > Garmin GPS > March 2006 > Re: 2 Questions: Etrex Legend Altimeter Accuracy and which GPS to
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Re: 2 Questions: Etrex Legend Altimeter Accuracy and which GPS to
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| Arthur Hass 2006-03-17, 5:48 pm |
| I have use a map76CS extensively making elevation profiles of running
courses. The barometric altimeter is much more accurate than reported
GPS altitude. However, the automatic calibration function does have
issues. Automatic calibration is dependent on GPS reception, so if you
have bad reception or get multi-path errors, the altitude accuracy is
degraded. You can see the error in downloaded & graphed profiles. I
have logs with jumps of 20 feet on essentially level ground. Of course,
20 feet is not a large error compared to that in GPS altitude readings
and wouldn't be important for most uses.
If the automatic calibration is turned off, these instances are almost
non-existent. So, if I'm making a profile over a short period of time,
I calibrate the altimeter at a known elevation and set automatic
calibration to off.
The barometric altimeter is also affected (10 foot jumps) by simply
pressing the buttons and rocker key on the map76CS. This is apparent in
downloaded track logs - especially at the beginning and end when I've
been pressing buttons. This happens regardless of how the automatic
calibration is set.
Arthur Hass
Reston, VA
Jesse wrote:
> I have an Etrex Legend and I have always been wondering about it's
> accuracy when it comes to altitude. Does anyone know how INaccurate it
> is?
>
> In addition I am thinking of getting another GPS with SiRF. I really
> like the feature of the barometric altimeter and electronic compass,
> but I have a manual very good compass and I don't know if the accuracy
> of the altimeter makes that much of a difference since I am a not a
> mountain climber. But, I really like the all in one, but there is a
> price issue attached to the higher-end model.
>
> Any suggestions?
>
> Thanks
>
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| Arthur Hass 2006-03-18, 5:48 pm |
| There are options. By letting the unit use the GPS altitude (not bad if
you have a WAAS lock), by entering a known altitude, or by entering a
known pressure.
Arthur Hass
Reston, VA
Jesse wrote:
> Thanks for everyone's feedback. BTW, how do you calibrate the
> barometric altimeter?
>
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| Dale DePriest 2006-03-18, 11:48 pm |
| Every time the weather changes. Generally every day before you use it.
Dale
Jesse wrote:
> Hmm. How often does one have to recalibrate?
>
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs
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| Arthur Hass 2006-03-19, 5:48 pm |
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Jesse wrote:
> dold@XReXX2XQue.usenet.us.com wrote:
> :
> : Over time, the auto calibration seeks to bring the indicated altitude
> : in accordance with the GPS altitude.
>
> Does this mean that it will be the same as the GPS [estimated] altitude?
>
No, it uses the GPS alt. to recalibrate - not sure how. But, it seldom
re-calibrates in my experience. However, when it does, there is usually
a spike (10-20 feet) in the elevation profile.
> :: If the automatic calibration is turned off, these instances are
> :: almost non-existent. So, if I'm making a profile over a short
> :: period of time, I calibrate the altimeter at a known elevation and
> :: set automatic calibration to off.
>
> If one has the GPS unit off and calibration off, does one have to
> calibrate it again?
Yes.
> :
> : If you are sitting in one spot with good GPS reception, might you just
> : accept the displayed altitude and turn calibration off?
>
> With the new SiRF units, you will tend to get good reception so the BA
> is redundant then?
Don't know. I don't have that unit.
> :
> : Someone who is used to using any sort of barometric altimeter might be
> : impressed with the altimeter in a GPS, but I was dismayed to find
> : that it needs calibration far more often than I ever would have
> : believed.
> :
> : I have a GPSMap 60cs, and I recently made a track where an elevated
> : highway appears to be below sea level near the San Francisco bay.
>
> It sounds like it's using the GPSs altitude since this is a similar
> response I get with me Etrex Legend.
This could happen easily if the auto-calibration has only been turned on
a short time, or if the unit was not calibrated when turned on - even if
it was set for auto-calibrate. It takes an hour or more to hone in on
the correct altitude when set to auto-calibrate (if off significantly
when turned on and not calibrated).
Arthur Hass
Reston, VA
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| Arthur Hass 2006-03-19, 5:48 pm |
| There is a setting to allow calibration when the unit is off, but it
uses the battery and I believe the setting need to be turned off when
changing batteries (there is a warning about this in the manual). I've
never used this.
Jesse wrote:
> ***Correction: If one has the GPS unit off and calibration ***ON***,
> does one have to
> calibrate it again?
>
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